It is far easier to distinguish between accidents and substance than to give an exhaustive list of the ultimate and irreducible classes of the former. Aristotle enumerates _nine_: Quantity (ποσόν), Quality (ποῖον), Relation (πρὸς τι), Action (ποιέιν), Passion (πάσχειν), Where (ποῦ), When (ποτέ), Posture (κεῖσθαι), External Condition or State (ἔχειν). Much has been said for and against the exhaustive character of this classification. Scholastics generally have defended and adopted it. St. Thomas gives the following reasoned analysis of it:(220) Since accidents may be distinguished by their relations to substance, we see that some affect substances intrinsically, others extrinsically; and in the former case, either absolutely or relatively: if relatively we have the category of _relation_; if absolutely we have either _quantity_ or _quality_ according as the accident affects the substance by reason of the matter, or the form, of the latter. What affects and denominates a substance extrinsically does so either as a cause, or as a measure, or otherwise. If as a cause, the substance is either _suffering_ action, or _acting_ itself; if as a measure, it denominates the subject as in _time_, or in _place_, or in regard to the relative position of its parts, its _posture_, in the place which it occupies. Finally, if the accident affects the substance extrinsically, though not as cause or as measure, but only as characterizing its external condition and immediate surroundings, as when we describe a man as clothed or armed, we have the category of _condition_. It might be said that all this is more ingenious than convincing; but it is easier to criticize Aristotle’s list than to suggest a better one. In addition to what we have said of it elsewhere,(221) a few remarks will be sufficient in the present context. Some of the categories, as being of lesser importance, we may treat incidentally when dealing with the more important ones. _Ubi_, _Quando_, and _Situs_, together with the analysis of our notions of Space and Time, fall naturally into the general doctrine of _Quantity_. The final category, ἔχειν, however interpreted,(222) may be referred to _Quality_, _Quantity_, or _Relation_.